The unique properties of graphene have recently attracted major attention leading to proposals in electronic,
optoelectronic, and detector applications. Micro-Raman spectroscopy has been utilized as a convenient tool for
identifying graphene layers. Most Raman studies were limited to layers on silicon substrates with an oxide layer
thickness of 300 nm, rendering graphene visible under an optical microscope. The development of graphene technology
requires its integration with different materials and strict control of the number of layers and defects. Thus it is
important to extend the nanometrology capabilities of Raman spectroscopy to arbitrary substrates and temperatures.
Here we report that the deconvolution of the 2D band allows one to count graphene layers even when placed on
"inconvenient" substrates such as glass or sapphire. We also show that even small excitation laser power typically used
for Raman spectroscopy may lead to strong heating in graphene. The determined temperature coefficients for graphene
allowed us to evaluate the temperature rise and decouple the temperature effects from those due to variations in the
graphene edges or substrates.
An inexpensive, easily integrated, sensitive photoreceiver operating in the communications band with a 50-GHz
bandwidth would revolutionize the free-space communication industry. While generation of 50-GHz carrier AM or FM
signals is not difficult, its reception and heterodyning require specific, known technologies, generally based on silicon
semiconductors. We present a 50 GHz photoreceiver that exceeds the capabilities of current devices. The proposed
photoreceiver is based on a technology we call Nanodust. This new technology enables nano-optical photodetectors to
be directly embedded in silicon matrices, or into CMOS reception/heterodyning circuits. Photoreceivers based on
Nanodust technology can be designed to operate in any spectral region, the most important to date being the
telecommunications band near 1.55 micrometers. Unlike current photodetectors that operate in this spectral region,
Nanodust photodetectors can be directly integrated with standard CMOS and silicon-based circuitry. Nanodust
technology lends itself well to normal-incidence signal reception, significantly increasing the reception area without
compromising the bandwidth. Preliminary experiments have demonstrated a free-space responsivity of 50 &mgr;A/(W/cm2),
nearly an order of magnitude greater than that offered by current 50-GHz detectors. We expect to increase the Nanodust
responsivity significantly in upcoming experiments.
Nanostructures, such as quantum dots, nanocrystals and nanowires, made of wurtzite ZnO have recently attracted
attention due to their proposed applications in optoelectronic devices. Raman spectroscopy has been widely used to
study the optical phonon spectrum modification in ZnO nanostructures as compared to bulk crystals. Understanding the
phonon spectrum change in wurtzite nanostructures is important because the optical phonons affect the light emission
and absorption. The interpretation of the phonon peaks in the Raman spectrum from ZnO nanostructures continues to be
the subject of debates. Here we present a comparative study of micro-Raman spectra from ZnO quantum dots,
nanocrystals and nanowires. Several possible mechanisms for the peak position shifts, i.e., optical phonon confinement,
phonon localization on defects and laser-induced heating, are discussed in details. We show that the shifts of ~2 cm-1 in
non-Resonant spectra are likely due to the optical phonon confinement in ZnO quantum dots with the average diameter
of 4 nm. The small shifts in the non-Resonant spectra from ZnO nanowires with the diameter ~20 nm - 50 nm can be
attributed to either defects or large size dispersion, which results in a substantial contribution from nanowires with
smaller diameters. The large red-shifts of ~10 cm-1 in the resonant Raman spectrum from nanocrystals were proved to be
due to local laser heating.
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